An optical waveguide formed in, for example, an optical element is a structure that confines light within the optical waveguide and leads the light to a destination depending on the difference in refractive indexes of a guide portion and a surrounding portion. Materials used for an optical waveguide have a large refractive index and are influenced by thermal expansion or the thermo-optic effect such that the size or the refractive index is changed. For this reason, a measure to keep the optical waveguide at a constant temperature is generally taken.
An optical element having an optical waveguide, especially a second-harmonic generator, is made of a crystalline material such as lithium niobate (LiNbO3: LN) and lithium tantalite (LiTaO3: LT). The crystalline material is periodically poled for having a wavelength conversion function. This element is often combined with a laser diode (LD). The poling frequency depends on a wavelength to be converted and thus when the poling frequency becomes shorter or longer due to the temperature, the wavelength conversion efficiency degrades.
Thus a temperature control for keeping the optical waveguide at a constant temperature is adopted. For example, Patent Document 1 discloses that a thin film heater is formed on an optical waveguide and warms the optical waveguide so that the temperature of the optical waveguide is kept constant irrelevant of surrounding conditions and the wavelength conversion efficiency of the second-harmonic generator does not degrade.    Patent Document 1: Japanese Patent Application Laid-Open Publication No. 11-326966 (page 6, FIG. 1)